Magnetic memories, particularly magnetic random access memories (MRAMs), have drawn increasing interest due to their potential for high read/write speed, excellent endurance, non-volatility and low power consumption during operation. An MRAM can store information utilizing magnetic materials as an information-recording medium. One type of MRAM is a spin transfer torque random access memory (STT-RAM). STT-RAM utilizes magnetic elements written at least in part by a current driven through the magnetic element.
For example, FIG. 1 depicts one exemplary magnetic tunneling junction (MTJ) 10 as it may be used in a conventional STT-RAM. The conventional MTJ 10 is typically formed over a bottom contact 11, uses conventional seed layer(s) 12 and includes a pinning layer, e.g., a conventional antiferromagnetic (AFM) layer 14, a conventional pinned layer (or a reference layer) 16, a conventional tunneling barrier layer 18, a conventional free layer 20, and a conventional capping layer 22. Also shown is a top contact 24.
Conventional contacts 11 and 24 are used in driving the current in a current-perpendicular-to-plane (CPP) direction, or along the z-axis as shown in FIG. 1. The conventional tunneling barrier layer 18 is nonmagnetic and is, for example, a thin insulator such as MgO. The conventional seed layer(s) 12 are typically utilized to aid in the growth of subsequent layers, such as the AFM layer 14, having a desired crystal structure. Direct exposure of the conventional free layer 20 to the top contact 24 may result in a disordered interface, dead magnetic regions and enhanced damping. Consequently, the conventional capping layer 22 is provided directly on the free layer 20, prior to deposition of the top contact 24. This conventional cap acts as a diffusion block and improves the surface quality of the conventional free layer 20. The conventional pinned layer 16 and the conventional free layer 20 are magnetic. The magnetization 17 of the conventional pinned layer 16 is fixed, or pinned, in a particular direction, typically by an exchange-bias interaction with the AFM layer 14. Although depicted as a simple (single) layer, the conventional pinned layer 16 may include multiple layers. For example, the conventional pinned layer 16 may be a synthetic antiferromagnetic (SAF) layer including magnetic layers antiferromagnetically or ferromagnetically coupled through thin conductive layers, such as Ru. In such a SAF, multiple magnetic layers interleaved with a thin layer of Ru may be used.
The conventional free layer 20 has a changeable magnetization 21. Although depicted as a simple layer, the conventional free layer 20 may also include multiple layers. For example, the conventional free layer 20 may be a synthetic layer including magnetic layers antiferromagnetically or ferromagnetically coupled through thin conductive layers, such as Ru.
Spin transfer torque may be used to write to the conventional MTJ 10. In particular, spin transfer torque rotates the magnetization 21 of the conventional free layer 20 to one of the two directions along its easy axis. When a write current is passed through the conventional MTJ 10 perpendicular to the plane of the layers, electrons may be spin polarized by transmission through or reflection from the conventional pinned layer 16. The spin transfer torque on the magnetization 21 of the conventional free layer 20 may be adequate to switch the conventional free layer 20 if a sufficient current is driven through the conventional MTJ 10. Therefore, the conventional free layer 20 may be written to the desired state. The conventional MTJ 10 may thus be used for data storage in an STT-RAM.